This paper studies communication outages in Multiple-Input Multiple-Output (MIMO) block-fading channels with imperfect Channel State Information at the Receiver (CSIR). Using mismatched decoding error exponents, the authors prove the achievability of the generalized outage probability, the probability that the Generalized Mutual Information (GMI) is less than the data rate, and show that this probability is the fundamental limit for Independent and Identically Distributed (i.i.d.) codebooks. Then, using nearest neighbor decoding, they study the generalized outage probability in the high Signal-to-Noise Ratio (SNR) regime for random codes with Gaussian and discrete signal constellations.